1. Field of the Invention
An object of the invention is a light detection circuit which can be used to detect the appearance of an illumination. This circuit can be used preferably in the field of electronic integrated circuits where, if it is integrated into the same substrate as an integrated circuit to be protected, it can take into account an illumination to which this circuit would be subjected.
2. Description of the Prior Art
Certain electronic integrated circuits are confidential in character, either because the information that they memorize should remain secret or because their operation must remain concealed. The latter case generally comes up when it sought to prevent forgeries or else, especially with memory card, to prevent the reconstitution of secret algorithms. For, it has been realized that the operation of an integrated circuit can be known and understood by deduction when the passivation layer which covers this circuit at the end of its manufacture has been removed. To this end a selection is made, after depassivation, of the functions of the circuit while observing the flow of current in the connections of this circuit with an electron microscope. The flow of current causes localized mechanical constraints in semi-conducting parts. From this, functional information can be deduced about the integrated circuit.
In memory cards, the integrated circuits used are generally of the non-volatile type, the memory cell of which comprises a floating gate transistor. Some of them are programmable and erasable through the subjecting of the chip to ultraviolet radiation. This radiation is spectrally located outside the visible band. At the end of the manufacture of these circuits, the chips are covered with a transparent passivation layer by which, after a prior operation to test the proper working of the integrated circuit, the recorded test information is erased and the memory is returned to the state of a virgin memory for use as desired by the purchaser. As soon as this testing operation is completed, the integrated circuits which have thus been passivated are buried in opaque holding supports. In memory cards, especially of the bank card type, this opaque support may consist of the plastic material of the card itself. The problem to be resolved consists in preventing a dishonest person from removing the semi-conducting chip from the card after it has been programmed by the user, in order to examine it with an electron microscope in an attempt to deduce its operation therefrom or at least to neutralize its access codes.
Certain integrated circuits, especially those comprising electrically erasable memories or memories designed to be programmed only once, comprise opaque passivation layers. For these circuits, through measurement of the illumination, it is ultimately the depassivation operation that it is sought to control.